1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device using ion implantation, and more specifically, to an ion implantation method using a resist mask and a method of removing the resist mask. Note that the ion implantation in the present invention indicates ion beam irradiation and includes both ion beam irradiation with mass separation and ion beam irradiation without mass separation.
2. Description of the Related Art
In steps of manufacturing a semiconductor element, introduction (doping) of an impurity element to semiconductor by ion implantation is performed for valence electron control. An ion implantation method is roughly described as a method of dissociating a gas including an element-belonging to group 13 or group 15 of the periodic table, such as diborane (B2H6) or phosphine (PH3), to ionize it and accelerating the ion by an electric field to physically implant it into a substrate on which a semiconductor element is formed. With respect to ionic species produced at this time, it is known that plural species are produced when they are classified by mass number. A method of mass-separating these ions and implanting a single ion is generally called the ion implantation method. Also, a method of implanting an ion without particularly performing mass separation is called an ion dope method, a plasma dope method, or the like.
In all cases, it is necessary to selectively implant an ion into a predetermined region of a semiconductor to form an impurity region. Thus, a method of forming a resist pattern in advance to use it as a mask is used. Hereinafter, ion implantation using a resist as a mask and removal of the resist used as the mask will be described.
FIG. 4 shows a step of forming source and drain regions of TFTs of a CMOS structure. Since it is necessary to selectively introduce phosphorus (P) into source and drain regions 105 of an n-channel TFT 111, a p-channel TFT 110 is masked by a resist 107. When ion implantation is performed with this state, an ion including P is implanted into the source and drain regions 105 of the n-channel TFT 111 through a gate insulating film 102 using a gate electrode 101 as a mask.
Subsequently, the process advances to a step of removing the resist 107 as the mask. A method of removing the resist using a special stripper or a method of ashing (etching) the resist by supplying an oxygen radical is generally used. An ashing step is performed using high frequency (RF) plasma with parallel flat plates, high density plasma process with inductive coupled plasma (ICP), microwave excitation plasma, or the like.
However, there is a problem that the resist cannot be easily removed in the step of removing the resist. In ion implantation, since an ion is accelerated by a high electric field to implant it into a substrate, when ion collision is caused, the kinetic energy is converted into heat energy to heat the substrate and objects formed thereon. Although the heated temperature depends on an ion implantation condition, it is known that the temperature can rise up to about 200° C.
In contrast to this, only baking process at about 100° C. to 140° C. is performed for the resist formed as the mask. Thus, when the resist is substantially heated at a temperature higher than that temperature, a chemical change such that the surface thereof is altered and cured is produced. Generally, the thus altered resist cannot be removed even by using the special stripper. Thus, ashing having a stronger removal effect is generally used.
Further, when a P ion is implanted, P reacts with a novolac resin as a resist material so that an altered layer in which a structure is changed is produced. Since the altered layer is chemically stable, it cannot be easily removed by a conventional stripper or a conventional ashing method. When a dose of P is large, such a tendency becomes remarkable. Thus, ashing process for a very long time is required. Also, since the resist cannot be completely removed by only ashing process, there is a case where a residue thereof is produced, causing inconvenience in later steps.